FIG. 1 is a local longitudinal sectional view of a semiconductor device having a conventional Schottky junction electrode. The semiconductor device is reported, for example, in the Reference, IEEE Trans. Microwave Theory Tech. (Vol. No. 46, No. 6, Page 756, 1998) authored by U. K. Mishra et al.
As shown in FIG. 1, the semiconductor device is a heterojunction field-effect transistor which has a multilayered structure of a plurality of nitride compound semiconductor layers formed on a sapphire substrate. Specifically, a buffer layer 62 comprising aluminum nitride (AlN), a gallium nitride (GaN) channel layer 63, and an aluminum gallium nitride (AlGaN) doped layer 64 are sequentially formed on the sapphire substrate 61, and the laminated body of the nitride compound semiconductors is thus configured on the sapphire substrate 61.
In addition, a source electrode 6S and a drain electrode 6D are formed in contact with the AlGaN electron supplying layer 64, and these source electrode 6S and the drain electrode 6D are in ohmic contact with the AlGaN electron supplying layer 64.
Furthermore, the source electrode 6S and the drain electrode 6D are spaced out, a gate electrode 67 is formed in contact with the AlGaN electron supplying layer 64, and this gate electrode 67 is in Schottky contact with the AlGaN electron supplying layer 64. In other words, the gate electrode 67 is a Schottky gate electrode. The gate electrode 67, herein, comprises a two-layer laminated structure: a Ni layer 671 in contact with the AlGaN electron supplying layer 64, and an Au layer in contact with the Ni layer 671. In the Schottky interface of GaN semiconductors which comprise GaN, AlGaN or the like, since the influence of Fermi level by pinning is small, the barrier height (f B) is determined by the difference between the work function (Wm) of a metal and the electron affinity (?s) of a semiconductor.f B=Wm−?s  (1)
Therefore, the Schottky junction electrode 67 of a semiconductor device by a prior art was in contact with the AlGaN layer 64, and comprised a metal layer 671 comprising metals of high work function, for example, Ni, Pt, Pd or the like. In addition, the Au layer 672 is formed on the metal layer 671 to reduce the resistance of the electrode. If the Schottky junction electrode 67 comprises Ni, Pt and Pd, even though a high Schottky barrier can be obtained, a problem exists in that the barrier is thermally unstable, such as the transposition point of Ni being low, approximately 353° C., for example. With a semiconductor device using GaN as the principal material, operation at a high power density (1 to 10 W/mm) is possible because high current density (up to 1A/mm) and high dielectric strength (up to 100V) can be obtained. Under such operating conditions, since the temperature in the vicinity of the gate electrode rises to over 400° C. accompanied by self-heating, the thermal diffusions of Ni, Pt and Pd which are in Schottky contact with the GaN semiconductor and the alloying reaction between Au which constitutes the metal layer 672 and Ni, Pt and Pd were significant.
In order to confirm these phenomena, heat treatment was performed for 15 minutes on the conventional semiconductor device shown in FIG. 1. FIG. 2 is a diagram showing the reverse directional gate current-voltage characteristics measured before and after heat treatment was performed on the semiconductor device shown in FIG. 1. In FIG. 2, the vertical axis indicates the gate current (A/mm) and the horizontal axis indicates the gate-drain voltage (V). According to FIG. 2, it was confirmed that the reserve directional gate current to the gate-drain voltage was increased by about a single digit by performing heat treatment on the conventional semiconductor device shown in FIG. 1.
Moreover, the distribution of the constituent elements in depth direction before and after heat treatment of the conventional semiconductor device shown in FIG. 1 was examined by using Auger spectroscopy. FIG. 3 is a diagram showing the Auger profile before heat treatment of the conventional semiconductor device shown in FIG. 1. FIG. 4 is a diagram showing the Auger profile after heat treatment of the conventional semiconductor device shown in FIG. 1. In FIG. 3 and FIG. 4, the vertical axis indicates the Auger strength (a. u.) and the horizontal axis indicates the sputtering time (minute). By comparing FIG. 3 with FIG. 4, it was confirmed that the interdiffusion of Ni and Au was generated by performing heat treatment at 500° C. on the conventional semiconductor device shown FIG. 1. Therefore, the increase of reverse directional gate current by the heat treatment, as shown in FIG. 2, is considered to be due to the deterioration of the Schottky barrier at the interface with the AlGaN electron supplying layer 64 because the interdiffusion of Ni and Au occurred, as shown in FIG. 3 and FIG. 4, thereby promoting the alloying of Ni and Au, and the work function of the NiAu alloy was smaller than that of Ni. Additionally, there was a problem in that, at high temperatures, thermal diffusion of Ni comprising the Schottky junction electrode 671 to the AlGaN electron supplying layer 64 occurs, forming a deep level, and thereby, destabilizing element characteristics.